1. Field of the Invention
This invention relates generally to the device configuration and processes for manufacturing inductor coils. More particularly, this invention relates to an improved configuration, process and materials for manufacturing compact inductor coils applicable for accurate current measurements.
2. Description of the Prior Art
For those of ordinary skill in the art, an inductive coil is usually not suitable for current measurement due to the variation of resistance with temperature. Specifically, an inductive coil is generally made with copper coils. Since the copper has a relative high temperature coefficient of resistance (TCR), as the current passes through the copper coils, the coils experience a temperature rise. A higher temperature in turn causes a higher resistance in the coils with a positive TCR. The variation of the resistance in turn causes a change in the current conducted in the coils. For these reasons, in order to measure a direct current conducted in the coils, a separate resistor that is serially connected to the coils is often required.
Additionally, the configurations and the process of manufacturing a high current inductor coil are still faced with technical challenges that inductor coils manufactured with current technology still does not provide sufficient compact form factor often required by application in modern electronic devices. Furthermore, conventional inductor coils are is still manufactured with complicate manufacturing processes that involve multiple steps of epoxy bonding and wire welding processes.
Shafer et al. disclose a high current low profile inductor in a U.S. Pat. No. 6,204,744, as that shown in FIG. 1. The inductor disclosed by Shafer et al. includes a wire coil having an inter coil end and an outer coil end. A magnetic material completely surrounds the wire coil to form an inductor body. First and second leads connected to the inner coil end and the outer coil end respectively extend through the magnetic material to the exterior of the inductor body. As shown in FIG. 1, the inductor coil 10 is mounted on a circuit board 12. The inductor coil 10 includes an inductor body 14 that has a first lead 16 and a second lead 18 extending outwardly from the coil 10. The leads 16 and 18 are bent and folded under the bottom of the inductor body 14 and are shown soldered to a first pad and a second pad 20, 22 respectively. As shown in FIG. 1B, the inductor 10 is constructed by forming a wire coil 24 from a flat wire having a rectangular cross section. By forming the wire into a helical coil. The coil 24 includes a plurality of turns 30 and also includes an inner end 26 and an outer end 28. A lead frame 32 that includes a first lead 16, which has one end 34, welded to the inner end 26 of the coil 24. The lead frame also includes a second lead 18 which has one end 38 welded to the outer end 28 of coil 24. The leads 16 and 18 include free ends 36, 40, which are attached to the lead, frame 32. A resist welding process is applied to weld of ends 34, 38 to the inner end 26 and the outer end 28 of coil 24.
The inductor coil as shown in FIGS. 1A to 1C by Shafer et al. still have several limitations. As the wire coil 24 formed by flat wires that has stand on a vertical direction, the height of the flat wire 24 becomes an inherent limitation to the form factor of the inductor coil. Further miniaturization of the inductor coil becomes much more difficult with a vertical standing flat wire as shown in FIG. 1B. The production cost is also increased due to the requirements that the lead frame and the coil must be separately manufactured. The manufacture processes are further complicated as several welding and bonding steps are required to securely attach the welding ends of the flat wire to the welding points of the lead frame. The production yields and time required to manufacture the inductor coil are adversely affected due to the more complicate inductor configurations and multiple boding and welding manufacturing processes.
Japanese Patent Applications 2003-229311, and 2003-309024 disclose two different coil inductors constructed as conductor rolled up as an inductor coil. These inductors however have a difficulty that the inductor reliability is often a problem. Additionally, the manufacturing methods are more complicate and the production costs are high. The high production costs are caused by the reasons that the configurations are not convenient by using automated processes thus the inductors as disclosed do not enable a person of ordinary skill to perform effective cost down in producing large amount of inductors as now required in the wireless communications.
In addition to above discussed limitations, conventional inductive coils typically composed of copper that has low resistance. However, copper has a relatively large value of temperature coefficient of resistance (TCR), e.g., the TCR is about +4,300 ppm/deg. As the current passes through the inductive coil, the temperature of the inductive coil increases, thus changes the value of the resistance and that in turn changes the current passing through the inductive coil. A measurement of current may therefore incur a 0.43% error when there is one degree of change in temperature. In order to correct this potential error of current measurement, conventional techniques of measuring current conducted in the inductive coils further requires a separate resistor connected to the inductive coils as shown in FIG. 1D. FIG. 1D shows an equivalent circuit of an inductive coil 60 implemented with a voltage converter 70. In order to measure a current passing through the circuit, a separate resistor 80 of low resistance must be employed. This requirement of using a separate resistor leads to more complicate manufacturing processes, higher production costs and lower production yields.
Therefore, a need still exists in the art of design and manufacture of inductors to provide a novel and improved device configuration and manufacture processes to resolve the difficulties. In order simplify the implementation configuration with reduced cost; it is desirable to first eliminate the requirement of using a separate resistor for current measurement. It is desirable that the improved inductor configuration and manufacturing method can be simplified to achieve lower production costs, high production yield while capable of providing inductor that more compact with lower profile such that the inductor can be conveniently integrated into miniaturized electronic devices. It is further desirable the new and improved inductor and manufacture method can improve the production yield with simplified configuration and manufacturing processes.